Keep in mind the cost consists of whatever is financed plus whatever isn't.
21st (last I checked) charges the customer invoice plus 10%, of which you get 5% back as the dealer. You pay for whatever, get reimbursed by 21st (on their schedule?) or deal with paperwork to have them cut the checks directly. In either case the customer finances it (unless the home cannot be sold or it comes back to you).
invoice on a home like you describe might be under $20k, but frankly the amount of bang-for-the-buck is really high in this range, I think. Then add on $5k for move and set, (perhaps), $5k for a central A/C (if you're doing that), $5k for installation, skirting and deck, $5k for electric, gas, water and sewer hookup, some slush in there, and you're at around $40k which can be financed (to qualified buyers) for a reasonable amount monthly plus 10% down. The 10% down is what's paying your 10% overhead (5% kickback and 5% profit for 21st) and the rest you are guaranteeing anyway so it's just like you did it yourself but someone else floats the cash out of pocket (for one year) and deals with the loan origination (Dodd Frank) paperwork for you.